This invention relates generally to a high definition television (HDTV) system and specifically to a digital spectrum compatible (DSC) HDTV system.
Zenith Electronics Corporation, which previously announced its spectrum compatible high definition television system, recently announced its digital implementation thereof, which is called DSC-HDTV. In this system, encoded video data is formatted in a transmission frame having a plurality of successive data segments, each comprising a series of multi-level data symbols, including a predetermined data segment sync character. The data segment sync character, which establishes the timing of the data segments, is selected such that it produces a zero or reference level between higher and lower levels at three successive sampling points in the received television signal. The repetitive nature of the data segments and their timing enables ready detection of the data segment sync characters without requiring an excessive amount of data space in each data segment. That invention is described and claimed in copending application Ser. No. 894,388 above.
As fully described in U.S. Pat. No. 5,086,340, referred to above, HDTV receivers preferably employ special linear filters for minimizing the effects of NTSC co-channel signals. The linear filter has null points or notches that correspond to the NTSC signal frequencies that have the greatest interference-causing potential and a notch at DC, which has a benefit in that any direct current components acquired by the signal are precluded from the receiver. Thus, the benefits of the linear filter in the receiver are the rejection of NTSC signal interference and DC rejection. The disadvantages are a 3 dB signal-to-noise (S/N) loss and some corruption of the data. To preserve the data, it is modulo N precoded in the transmitter. The receiver must therefore incorporate suitable postcoding circuitry to reconvert the data. Modulo N precoding may be accomplished by a modulo N adder and a delay circuit in a feedback arrangement and/or by other suitable circuitry. An N level input signal thus yields an N level output signal.
The N level input to a linear filter, consisting of an algebraic adder and a delay circuit in a feed forward configuration, results in a greater than N level output signal. The number of levels is dependent upon the number of feed forward taps on the delay unit. For a single tap delay, the output is given by the formula 2N-1. The linear filter must be followed by an appropriate "slicer" to yield the correct N level data signals.
Where there is no significant NTSC co-channel interference, the linear filter 3 dB S/N loss may be avoided by processing the precoded modulo N signal through a suitable modulo N postcoder in the receiver to again produce the original N level data signal. It should be apparent that in a benign environment, or one that is devoid of NTSC co-channel interference, such as a cable system, the linear filter is not used.
The transmission system is useful for both terrestrial broadcasting (as described above) and for cable systems. As mentioned, the relatively benign environment of a cable system obviates the need for the linear filter since no NTSC co-channel is experienced and the transmission medium is generally much less noisy. Therefore a data constellation having a greater number of levels (i.e. larger N) may be used for a greater data rate. The principles however are the same.
The transmission frame preferably also includes selected field sync signals. In one embodiment, the field sync or timing signals are 2 level symbols and the data is, for example, in multilevel symbol form, e.g. either 2, 4, 8 or 16 level VSB (vestigial sideband) symbols. The levels of the 2 level symbols may comprise a subset of the multilevel symbols. Thus, if four levels (a), (b), (c) and (d) are used, levels (a) and (c) may be used for 2 level data, all four levels (a), (b), (c), (d) may be used for 4 level data and the two outermost levels (a) and (d) used for the 2 levels of the sync information or timing signals. Such an arrangement yields a robust sync signal, which is highly desirable. As will be seen, the S/N loss introduced by the linear filter in the receiver is acceptable with the more robust 2 outer level sync signal. The interference rejection of the linear filter enables the receiver to stay locked even under very weak and noisy signal conditions.
In another embodiment, the levels of the 2 level data and the synchronizing information may be between or intermediate the two upper and two lower levels of the 4 level data, i.e. between levels (a) and (b) and between levels (c) and (d). In the copending application 894,388 the level intermediate levels (a) and (b) is referred to as level (e) and the level intermediate levels (c) and (d) is referred to as level (f). Thus there are in reality six distinct levels. While the sync in this embodiment is not as robust as sync symbols having the 2 outer levels (a) and (d), the arrangement provides an average pilot which is equal for both 2 level and 4 level data and causes less interference into an NTSC co-channel.
The presently preferred embodiment for terrestrial broadcasting is an 8 level VSB transmission system, whereas for cable systems, the data may be transmitted as 16, 8, 4 or 2 level symbols, depending upon the noise characterizing the system. In a proper environment, 24 level data symbols are quite feasible for an even higher data rate. In the terrestrial transmission embodiment, the 2 level sync symbols preferably comprise levels L2 and L7 (FIG. 3) of the 8 level data symbols, and in the cable transmission systems, the 2 level sync symbols preferably comprise levels P4 and P13 (FIG. 4) of the 16 level data symbols. It will be appreciated that other levels for the sync may be selected within the teachings of the invention.
The invention in copending application Ser. No. 893,486 provides for the selection of alternate signal processing paths in the receiver, for minimizing the S/N loss due to the linear filter, in environments that have no significant NTSC co-channel interfering signals. The selection circuit of that invention bypasses the linear filter in the receiver in situations where NTSC co-channel interference is not determined to be a problem and instead processes the received signal using a complementary postcoding filter.
The present invention is concerned with recovery of a field or frame timing signal from the transmitted data segments. The data segments are the same length, each comprising 684 symbols, with 525 data segments corresponding to a transmitted video frame. Each frame further comprises alternating fields of 262 and 263 data segments. Although the data segments and data fields may be transmitted at the NTSC horizontal and vertical rates, respectively, it will be understood that the data segments do not directly correspond to horizontal scanning lines in a television display system. It will also be understood that this frame structure may be modified to incorporate different numbers of symbols per data segment and data segments per frame or field (such as 313 segments per field and 836 symbols per data segment) as disclosed in copending application Ser. No. 175,061, filed Dec. 29, 1993.
One of the data segments in each video field defines a field timing signal that identifies the beginning of the field. The field timing signal, which may occupy only a portion of the data segment, is thus similar in function to the vertical timing signal in a conventional television system. Means are provided in the receiver for recreating a reference data segment that includes a reference field timing signal. This may be accomplished each data segment with a PROM or other suitable memory device, or preferably may be accomplished by developing the digital field reference timing signal on an as-needed basis using, for example, a pseudo-random sequence generator. Each data segment is detected. The portion (or portions) of each data segment that corresponds to the data segment that contains the field timing signal, is compared with the reference field timing signal in the reference data segment. The errors between corresponding symbols in the compared portions of the data segments are accumulated. The data segment in each field that exhibits the least number of symbol errors is assumed to contain the field timing signal and therefore to identify the start of the field and is used to establish the timing relationship for the receiver. A confidence counter is employed to stabilize the identification process. For enhanced stability, the embodiment includes a different field timing signal in each of the two successive fields of data segments. In this embodiment, a field reference generator may recreate the two different reference field timing signals for comparison with the appropriate portions of the alternating fields of data segments.